Rotary hydraulic machines such as motors and pumps conventionally include a housing, a rotatable shaft that extends from the housing for coupling to an external device, and a shaft seal within the housing surrounding the shaft for preventing leakage of hydraulic fluid along the shaft into or out of the housing. The seal conventionally includes a generally V-shaped annular resilient sealing ring or gasket having one leg urged by a garter spring into sealing engagement with the shaft. In the event of seal failure due to age or wear, etc., the fluid may leak along the shaft past the seal into or out of the housing, causing loss of fluid and potential contamination. If the pump is mounted on a transmission housing in gear box, the pump fluid could leak into the transmission, or the transmission fluid could leak into the pump.
In an effort to address this situation and provide an external indication of a need for seal repair, it has been proposed to provide a pair of annular resilient sealing rings (with garter springs) spaced from each other lengthwise of the shaft. The pair of sealing rings provide double protection against fluid leakage along the shaft. A fluid passage extends from between the seals to externally of the housing, and a pressure plug is disposed at the outer end of this passage to prevent entry of dirt. When the pressure of fluid that leaks past the first seal reaches the retention pressure of the passage plug, which may be on the order of 1 to 3 psig, the plug is upset and fluid drips from the housing for observation by a user to indicate a need for seal repair.
A general object of the present invention is to provide a shaft seal of the described character that exhibits enhanced reliability in providing pressure relief for fluid that leaks past the annular resilient seal, that provides an indication of a need for seal repair by leakage from the housing, that seals against entry of dirt and debris, and that eliminates any requirement for the passage plug as in the prior-art seal arrangement described above.